Draw text with Python in 3d coordinate system
Thx, I found an example in the MeasureIt addon and here is some code from my Fast Carve addon to draw 2d text:

rv3d = self._view_context.region_3d
region = self._view_context.region
pos_text = location_3d_to_region_2d(region, rv3d, self._center)

blf.position(2, pos_text[0], pos_text[1], 0)
blf.draw(2, "r: {0:.3f}".format(self._radius))